<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<form action = "post">
    <p>
        <span>用户名：</span>
        <input type="text" id = "username" />
    </p>
    <!-- 多选框的值就是定义好的value -->
    <p>
        <span>性别：</span>
        <input type = "radio" name = "sex" value = "man" id = "boy"/>男
        <input type = "radio" name = "sex" value = "woman" id = "girl"/>女
    </p>
</form>

<script>
    var input_text = document.getElementById("username");
    var boy_radio = document.getElementById("boy");
    var girl_radio = document.getElementById("girl");
    // 得到输入框的值
    // input_text.value;
    // 修改输入框的值
    input_text.value="admin";

    // 对于单选框，多选框等等固定的值，boy_radio.value只能取到当前的值
    boy_radio.checked = true;  // 赋值
    // girl_radio.checked;  // 查看返回的结果，是否为true，如果为true，则被选中。
</script>
</body>
</html>